268 EIGHTH REPORT — 1638. 



the internal part of the cast-iron cylinder. It was of such 

 tenuity as to take fire and burn at a candle like tinder. It is 

 here obvious that the iron and copper cylinders were heated 

 alike, but the latter expanded much more than the former, and 

 hence, on cooling, compressed the tin (still fluid, probably, about 

 the centre of the length of the cylinder, althougli cold at its 

 ends), and forced it out through the pores of the iron. The 

 limit of force here was only that of the cohesion of the copper' 

 cylinder. 



37. It is usually considered an ignorant prejudice of workmen, 

 that a " hard shin," as it is technically called, is given to cast 

 iron, after planing or turning, by coating it with oil, and, until 

 a short time since, I was myself of that opinion ; but, on exa- 

 mining some broken castings, whose surfaces had been turned 

 and exposed to oil for several years, I found that the oil had 

 penetrated the pores of the iron to a considerable depth. 



38. I also possess a piece of cast iron, of considerable thick- 

 ness, which formed, I believe, part of a furnace for decomposi- 

 tion of sea salt ; it contains throughout its mass a minute 

 quantity of chloride of sodium, and a great deal of sulphur. 

 This has been produced by cementation, in the same way as 

 Herapath describes an alloy of 



Zinc 92-6 



Iron 7-4= 100-0 



as being produced in the Bristol Zinc Works ; and as alloys of 

 cast iron with arsenic, antimony, and lead have been formed. 

 These observations are intended to show the importance of se- 

 lecting close-grained cast irons for works designed to resist 

 longest the action of air and water. 



39. Of the relative rates of corrosion of the various com- 

 mercial "makes," or specimens of wrought and cast iron, 

 scarcely anything is known, and that of a very general cha- 

 racter. It is certain that the blackest cast irons, viz. those 

 which contain the largest quantity of uncombined carbon or 

 graphite in a mere state of mixture, are acted on by air and 

 water the most rapidly. This has probably partly an electro- 

 chemical cause, and partly a mechanical one, from defects of 

 hardness, and open and porous grain. There can be little doubt 

 but that the suspended graphite in this kind of iron forms the 

 negative element of innumerable voltaic couples which aid the 

 process of oxidation. 



The gray or mottled iron most used for castings of ma- 

 chinery and engineering purposes in general, as containing a 

 less quantity of uncombined carbon, and having a denser 



